-- Eric Linder

How many cosmological parameters? posted 2/25/04
Constraining the dark energy dynamics with the CMB bispectrum posted 2/25/04
Self Calibration in Cluster Studies of Dark Energy posted 5/27/03
CMB and Supernova Constraints on Quintessence posted 5/27/03
Post-WMAP thoughts on Dark Energy, with Lyman alpha and LISA too posted 3/4/03
Cluster Number Count Surveys for Dark Energy posted 1/28/03
A Theoretician's Analysis of the Supernova Data posted 1/28/03
The Cosmological Constant and Dark Energy posted 8/6/02
Rethinking Lensing and Lambda posted 8/6/02
Can Clustered Dark Matter and Smooth Dark Energy Arise from Same Scalar Field posted 5/9/02
UCLA DM2002 conference Review posted 2/26/02
Constraints in Cosmological Parameter Space from the Sunyaev-Zel'dovich Effect posted 01/23/02
Supernovae, CMB, and Gravitational Leakage into Extra Dimensions posted 01/14/02
AAS conference review posted 01/14/02; updated 2/6/02
Measuring the Equation of State of the Universe: Pitfalls and Prospects posted 01/02/02
Measuring the Sound Speed of Quintessence posted 12/20/01
CfCP Dark Energy Workshop Review posted 12/18/01
Dimming Supernovae without Cosmic Acceleration posted 11/29/01; updated 12/21/01
Supernovae as a Probe of Particle Physics and Cosmology posted 12/01/01
Feasibility of Probing Dark Energy with Strong Gravitational Lensing Systems posted 11/04/01

How many cosmological parameters?
A.R. Liddle
astro-ph/0401198
Reviewed 02/25/04

This is a well written, timely article introducing some statistical rigor into the cottage industry of taking a public data set and proceeding to run a suite of model fits on it. Astro-ph has been plagued with a rash of the latter papers. As the iconoclast chef Adam on the TV show "Northern Exposure" once claimed in a trademark diatribe "you're going to make this yourself? Any one with a hot plate thinks they can cook." Recently anyone with a chi-square code thinks they can make an astonishing discovery on existing data.

Liddle provides the antidote. He explains the statistical information criteria clearly and emphasizes the cosmological applications, in particular the role of the number of parameters in the model. This is certainly relevant to issues such as introduction of a running coupling constant for the CMB power spectrum and dark energy models with several fitting parameters. It is natural to want to push the data as far as possible to learn more about our universe, but overinterpretation of measurements must be recognized. Liddle finds that when the number of parameters is factored in appropriately, the best current fit cosmology only involves five fundamental parameters. Of course this simplicity restricts the physics we can learn cosmologically, so we all look forward to the day when the wealth of new data allows us to make things more complex!

Constraining the dark energy dynamics with the CMB bispectrum
F. Giovi, C. Baccigalupi, & F. Perrotta
astro-ph/0308118
Reviewed 02/25/04

The CMB is such a powerful cosmological tool, due to its clear physics and precision observations, that it seems a great shame that it cannot play a larger role in elucidating dark energy. The power spectrum of the temperature anisotropies has a vague feel for a time averaged dark energy equation of state through the distance to a single redshift -- that of the last scattering surface. For measuring the time variation of the EOS, w', the CMB serves a use as breaking degeneracies between other cosmological parameters. On the largest angular scales, i.e. lowest multipoles, the ISW effect depends on the time variation and on any noncanonical sound speed of the DE but the ISW measurements are substantially obscured by unavoidable cosmic variance.

The pioneering work of Komatsu & Spergel and Verde & Spergel recognized that higher order correlations of the anisotropies could further break degeneracies. The primary CMB anisotropies are depressingly gaussian, causing the third order statistic -- the bispectrum -- to vanish. However, weak lensing of the CMB induces nongaussianities and at the same time essentially provides a crosscorrelation between late time, forming structure (the lenses) and early time CMB pertrubations (the sources). This interesting article by Giovi, Baccigalupi, & Perrotta extends the analysis to the case of time varying dark energy EOS, looking for a signature of the w' characteristic in the bispectrum.

They indeed find one, comparing three models with the same present EOS but different w' and computing the multipole shift of a feature in the bispectrum. This shift is of order Delta l=50, an order of magnitude larger effect than in the power spectrum (from the shift in distance to last scattering). Other systematic effects leading to such a shift could be removed by comparing the power spectrum and bispectrum. Because the cross correlation peaks as the structure formation goes nonlinear, the effect focuses in on a particular redshift range, coincidentally roughly that when dark energy begins to be dynamically important. Thus the bispectrum in insensitive to the present EOS, rather the conditions at z=1-2 have the main effect.

So while this method does not see w' per se, rather measuring some \bar w(z=1-2), it may have the potential to open another window on the dark energy. It does depend somewhat on knowledge of the nonlinear mass power spectrum; it's early days yet. Inspired by this article, EL has found that this or similar methods offers one of the best leverages (at least in the "smooth" case) to distinguish modified gravity sources for the acceleration from dark energy models with the same expansion history (see astro-ph/0402503).

Self Calibration in Cluster Studies of Dark Energy
S. Majumdar and J.J. Mohr
astro-ph/0305341
Reviewed 05/27/03

Cluster surveys contain a variety of information, both astrophysical and cosmological. The two central difficulties lie in translating the direct observations into the more general parameters and in separating the two types. The concept of self calibration has offered promise in one step of this process, going from the observable distribution function to the mass. This article considers the addition of further information beyond the distribution, such as the mass power spectrum or detailed follow up observations of individual clusters.

They present a clear line of argument, concluding that the increased uncertainty in cosmological parameters on including calibration parameters can be reversed by incorporating the extra types of data. This is a valuable article in this regard, explicitly demonstrating both effects. However I worry about a few beetles in the pudding. Most generally, there is the mantra that systematics should not be ignored. Here they assume that there are none beyond the calibration, and that those are universal (independent of mass), and uncorrelated with cosmology. Also, there is the relevant point that when dealing with the sophisticated (and expensive) task of precision cosmology that the community must (and gradually does) evolve beyond dealing with a constant equation of state w. A constant w is virtually physically vacuous (pun intended). Large scale structure calculations can now readily implement a time variation w' in distance, growth of density, and simulations - see, e.g., Linder & Jenkins astro-ph/0305286.

More specific issues include the true utility of follow up. The masses are not directly observable, and the M_f mass they define assumes a specific mass profile and concentration. Moreover the covariance with cosmological parameters is unknown. The power spectrum information seems more robust, but one still worries about the role of bias, which is maybe not as trivial to obtain from N-body simulations as the authors assert (in particular self consistently from simulations that include the full range of cosmological and dark energy parameters).

So overall I would tend to most believe the numbers in the self-cal column of their Table 1, with some hope for self-cal+P_cl. These present excellent determination of the matter density and sigma8, but less inspiring constraints on (constant) w, e.g. around 0.1-0.2. [Note that their claim that WMAP gives w to 0.11 actually represents all CMB experiments plus supernova plus structure. Also, in Fig. 4 the Planck contours must be mistaken as the ellipse with w restricted to -1 has parts lying outside the ellipse without the restriction.]

One of the main conclusions I took away from the article was confusion over the role of DUET. According to the numbers, the best experiment will be SPT (more than Planck, and even more than DUET) - and this is the first one to gather data as well. The results make it appear that DUET is only really relevant for sigma8. But looking at these experiments from the outside, I could be mistaken. I look forward to learning more from articles such as this, and calculations that include the necessarily present systematics and time variation w', as the weak lensing method is beginning to do.

CMB and Supernova Constraints on Quintessence
R.R. Caldwell and M. Doran
astro-ph/0305334
Reviewed 05/27/03

This illustrates the CMB parameter degeneracies and regions of sensitivity very clearly for dark energy and cosmological characteristics. They also present a well chosen and bountiful array of dark energy models to consider. The authors use a 7 to 8 dimensional phase space, more comprehensive than many, though still without tensor contributions. It is especially nice that the degeneracies are not treated solely in terms of the distance to the last scattering surface, but rather take into account the detailed structure of the acoustic peaks and the ISW effect. Furthermore, they consider fluctuations in the dark energy, to some extent at least. Overall this paper is the sort that gets you thinking, which is the best kind.